1. Field of the Invention
This invention relates to an improved ink jet printing apparatus and for generating ink drops on demand having selectively variable size.
2. Description of the Prior Art
There have been known in the prior art ink jet printing systems in which a transducer is selectively energized to produce ink drops on demand. Extensive efforts have been made to improve reliability and enhance the print quality and resolution of drop-on-demand ink jet printing systems.
Dot matrix printing at a resolution of 240 pels per inch produces printing that approaches the print quality produced by engraved type. A spot size of 125 to 150 .mu.m is generally needed to give full area fill at a resolution of 240 pels per inch. For most commercially available papers, a spot size of 125 to 150 .mu.m requires that the nozzle diameter be of the order of 50 to 75 .mu.m.
Surface tension forces are indirectly proportional to the nozzle radius, so from this relationship it is apparent that a decrease in the nozzle dimension will increase the reliability of the drop generator as long as the nozzle does not clog. For most nozzle designs, the optimum reliability is obtained with nozzles having a diameter of the order of 30 to 50 .mu.m. Thus, in general, in order to simultaneously optimize print quality and reliability, it is desirable to obtain the maximum drop volume using the smallest nozzle for which clogging does not occur. However, for printing systems which require high quality printing, it is recognized that, to obtain these desirable characteristics, incompatible requirements are presented.
There have been attempts in prior art printing systems to produce larger than normal drops in the drop-on-demand mode from a nozzle of a particular size. One such system is disclosed in U.S. Pat. No. 3,946,398 in which the volume of ink in each drop is varied by adjusting the magnitude of the drive voltage pulse. Another system is disclosed in U.S. Pat. No. 4,281,331 to Tsuzuki et al in which the energy content of the transducer driving pulse determines the size of the ink drop.
In some cases systems of the above-described type produce drops having a variation in drop velocity along with the change in drop size which degrades print quality. Compensation for this variation in velocity has been attempted in U.S. Pat. No. 4,222,060 to Sato et al by varying not only the amplitude but also the effective timing of each of the voltage drive pulses so that the resulting ink drops reach the print medium at the desired location. This compensation method requires complex control circuits which are difficult to modify to include future improvements.
Another system is described in the commonly assigned copending application entitled "Gray Scale Printing With Ink Jet Drop-On-Demand Printing Head" by F. C. Lee et al, Ser. No. 413,039, filed Aug. 30, 1983, in which the transducer comprises a plurality of separately actuable sections. Control means is provided which is operable in response to the print data to selectively actuate a particular combination of one or more of the separately actuable sections of the transducer to produce an ink drop of one of a plurality of sizes as specified by the print data.